The present invention relates an apparatus for use in an electron microscope and to the detection of electron images by converting them into light images and transferring them onto an electronic light-imaging device, and more particularly to enhancing the resolution of such images while not sacrificing sensitivity.
Electron microscopes use a beam of accelerated electrons which pass through or are deflected by a sample to provide an electron image and/or diffraction pattern of the sample. To provide a record of these images and/or diffraction patterns, the electrons have been converted into light images using scintillator materials (e.g., single crystal YAG and phosphors), and the light images and/or patterns are then captured by an imaging sensor. A transfer optic, typically one or more optical lenses or a fiber optic plate, transfers the light image to the imaging sensor. While photographic film and cameras have long been used to capture such light images and/or diffraction patterns, charge-coupled devices (CCD) of the type originally developed for astronomy to read light images into a computer have found increasing use in this field. Such CCD cameras offer excellent resolution, sensitivity, linearity, up to 2,048xc3x972,048 pixels, are reusable, and make the image available for viewing within seconds of recording.
A conductive medium is typically coated onto the entrance surface of the scintillator to prevent the buildup of electrical charges and also to prevent the entry of light from external sources. When the transfer optic is a fiber optic plate, the scintillator is typically glued onto the fiber optic plate, and the plate is then coupled with optical coupling oil or glue to the imaging sensor.
The resolution of prior art devices is limited by a number of factors including the extent to which light generated at a particular spot on the scintillator is imaged onto a single pixel at the imaging sensor. Current image coupling devices lose resolution due to leakage (scattering) of light sideways, either in the scintillator, the transfer optic, or both. Such light scattering increases background noise and creates a xe2x80x9chaze,xe2x80x9d making it difficult to image objects generating only weak intensity light which are located near objects which generate stronger intensity light.
One solution to the problem of image resolution is suggested by Mooney et al, U.S. Pat. No. 5,635,720. There, a light absorptive layer is positioned on a scintillator to absorb reflected, scattered light from the scintillator and prevent that scattered light from reaching the imaging device. However, while improving resolution of the image, the sensitivity of the device is reduced because of the absorption of light.
Extra mural absorption materials such as glasses have been used to absorb stray light from optical fibers. However, typically, extra mural absorption glass is introduced as separate fibers into a fiber bundle (for example, every nth fiber is a substituted EMA fiber, where n is a number greater than  greater than 1). Such technique provides a statistical level of absorbed light, typically less than 10%, but does not provide the type of selective light absorption required in electron microscopy.
Accordingly, the need still exists in the art for an apparatus which will improve the ultimate resolution of image sensors used to record images from electron microscopes while not reducing the sensitivity of the apparatus.
The present invention meets that need by providing a resolution enhancement device which utilizes either high extra-mural absorbent optical fibers in the transfer optic, and/or which uses a transfer optic which is bonded to the scintillator without the use of any external glues or adhesives. The device provides improved resolution of electron images from electron microscopes while not reducing the sensitivity of the apparatus. This enable the device to be used to observe and image objects which generate only weak light intensity but which are positioned near other objects which generate stronger light intensity.
In accordance with one embodiment of the invention, an apparatus for improving the resolution of electron images is provided and includes an electron beam forming an electron image, a scintillator located in the path of the electron beam for converting the electron image into a light image, and an imaging sensor positioned to receive and record the light image. The apparatus further includes a transfer optic associated with the scintillator for transferring the optical image to the imaging sensor, the transfer optic comprising at least one optical fiber including a layer of cladding material. The at least one optical fiber is oriented lengthwise with respect to an optical axis of the apparatus, typically substantially parallel to the path of the electron beam. The at least one optical fiber includes a layer of light absorptive material on the layer of cladding material which attenuates at least a portion of off-axis light entering the transfer optic. By xe2x80x9coff-axisxe2x80x9d light, it is meant light which enters the transfer optic at an angle greater than the critical angle. Preferably, the transfer optic comprises multiple optical fibers packed in an array, typically a hexagonal array.
In a preferred embodiment of the invention, the imaging sensor is a charge-coupled device and the transfer optic is a fiber optic plate. The scintillator may be any scintillator material which has found use in this art including single crystal yttrium-aluminum-garnet as well as coatings of particulate phosphors. Preferably, the cladding material and the light absorptive layer have a difference in refractive indices of less than about 0.1.
A preferred environment for the present invention is in an electron microscope having a projection chamber through which an electron beam forming an electron image and/or diffraction pattern traverse. Such an apparatus includes a scintillator located in the path of the electron beam for converting the electron image into a light image, and an imaging sensor positioned to receive and record the light image. The apparatus further includes the transfer optic associated with the scintillator for transferring the optical image to the imaging sensor as described previously.
In another embodiment of the invention, an apparatus for improving the resolution of electron images is provided and includes an electron beam forming an electron image, a scintillator located in the path of the electron beam for converting the electron image into a light image, and an imaging sensor positioned to receive and record the light image. The apparatus further includes a transfer optic associated with the scintillator for transferring the optical image to the imaging sensor, with the at least one optical fiber being oriented lengthwise with respect to an optical axis of the apparatus.
In this embodiment, the transfer optic and the scintillator are bonded to one another in the absence of a bonding agent such as a glue or other adhesive. Such a glue layer increases light scattering due to refractive index mismatches at the scintillator/glue and glue/transfer optic interfaces. The present invention eliminates such a glue layer and instead directly bonds the scintillator and transfer optic to one another. In a preferred form, the transfer optic and the scintillator are bonded using optical contacting of the respective surfaces followed by heat treatment to form a virtually defect-free bond interface without the need for glues or other bonding agents. Also, preferably, the transfer optic and the scintillator have refractive indices which differ by less than about 0.1. This embodiment also finds use in an electron microscope.
In a further embodiment of the invention, the apparatus includes both of the first and second embodiments. That is, the apparatus includes both a transfer optic having a high extra-mural light absorptive layer around each optical fiber as well as having the transfer optic directly bonded to the scintillator. This combination of features provides an apparatus in which the ultimate resolution of an image sensor used to record the light images from the electron microscope is improved while not reducing the sensitivity of the apparatus. Optionally, the apparatus includes a light reflective layer positioned between the source of the electron image and the scintillator. A preferred location for the reflective layer is positioned on a surface of the scintillator. A suitable material for such a reflective layer is aluminum.
Accordingly, it is a feature of the present invention to provide resolution enhancement for image sensors which are used to record images from electron microscopes while not compromising or degrading the sensitivity of the apparatus. This, and other features and advantages of the present invention, will become apparent from the following detailed description, the accompanying drawings, and the appended claims.